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Bio 1B, Spring, 2008, Evolution section 1 of 3
Lecture 10
Updated 3/20/08 2:51 PM
 10 Macroevolution.
 Reading; 7th edition 482-488; 6th edition 476-481.
 Microevolution to macroevolution
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A basic idea of Darwinism and neo-Darwinism is that macroevolution is the result of
the accumulated effects of microevolution.
Darwin’s discussion of the evolution of the vertebrate eye is still regarded as correct
and representative of how complex characters evolve.
 Evolutionary novelties
 Most evolutionary novelties are greatly modified version of existing structures
• A few characteristics had to be true novelties. Early chordates, such as the lancelet,
had no limbs. In an early lineage leading to fishes, the first proto-limbs appeared.
Limbs in vertebrates are greatly modified versions of the first limb-like structures.
• The bones of the inner ear in mammals provide an example of a new structure that is
derived from an existing structure in an ancestor. They are derived from parts of the
jaw of species ancestral to mammals.
 Evolutionary novelties are often derived from repeated structures that become
differentiated.
• Alismatales is a group of monocots thought to be ancestral to orchids. Alismatales
flowers are radially symmetric. In contrast, petals of orchid flowers are extremely
differentiated.
• Multigene families have evolved from a single ancestral gene. For example, α- and
β-globin, which together form the hemoglobin molecule in mammals, are derived
from an ancestral globin gene. So are other members of the globin gene family.
 Origin vs. maintenance
• One question about evolutionary novelties is whether the conditions that favor their
continued presence are the same as the conditions that favored their origin.
• For the vertebrate eye, they probably did.
• For the wing in birds, possibly not. Dromaeosaurs were small dinosaurs that were
probably ancestral to birds. Fossils of dromaeosaurs show clear evidence of feathers
but without having wings capable of gliding or flight. Front limbs that later became
wings might have been favored because they helped regulate body temperature or
played a role in social interactions, and only later became useful for gliding and
flying.
• Characters that have changed function are called exaptations. Birds’ wings are
probably exaptations; eyes probably are not exaptations.
Bio 1B, Spring, 2008, Evolution section 2 of 3
Lecture 10
Updated 3/20/08 2:51 PM
 Major changes in form can be caused by relatively minor changes occurring
during growth and development.
 Heterochrony.
• Heterochrony can result in the appearance of several coordinated changes because of
a single change in the timing of some event during development. By looking only at
the adult phenotype, it is not always easy to decide what changes actually cause the
difference in adult morphology. Large difference might be the result of a simple
change, as in the salamander foot, Fig. 24.16.
• Paedomorphosis is one type of heterochrony. Some salamanders retain gills as adults.
Gills are a juvenile character is most species of salamander.
• Another example of how a subtle change during growth can cause a major change in
morphology is the evolution of internode distances in plants. Internode distance
depends on production of gibberellins and sensitivity to gibberellins during growth.
If none is produced or if cells are not sensitive, then internode distances will decrease
to almost zero, resulting in rosate plants. In general, the production of gibberellins
rather than the sensitivity has evolved in rosate plants.
 Homeotic genes.
• Homeotic or Hox genes control where specific structures are formed. They were first
discovered in Drosophila but are now known in many plants and animals. The
bithorax mutation in D. melanogaster is a mutation to a Hox gene that results in two
pairs of wings. Hox genes form a large multigene family.
• Changes in the locations where Hox genes are expressed during development were
important for the evolution of vertebrates. One difference between fish and tetrapods
is where on the limb bud a Hox gene is expressed, as illustrated in Fig. 24.18.
 Hox genes in vertebrates.
• There were two duplications of the Hox gene cluster which may have resulted from
two genomic duplications. The first duplication occurred about 520 mya and may
have been a critical step in the evolution of vertebrates, and the second, about 425
mya may have permitted an increase in morphological complexity of vertebrates, as
illustrated in Figure 24.19.
 Natural selection and species selection
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Natural selection causes changes in each lineage.
Species selection causes changes in the numbers of species with different properties.
Species selection results from differences in the rate of speciation and extinction.
Once a species makes an adaptive transition, the descendant species may have such
an advantage that they will increase their geographic range, colonize new habitats,
and evolve very rapidly. The result will be an adaptive radiation of species that share
the new adaptation.
Bats and anteaters provide an illustration of species selection. Powered flight evolved
only once in mammals, in the ancestor of bats. Bats have become very diverse and
abundant because their ability to fly makes them effective predators and allows them
to escape predation themselves. Bat species comprise roughly 25% of the 4000
species of mammals. They have a worldwide distribution and are abundant wherever
Bio 1B, Spring, 2008, Evolution section 3 of 3
Lecture 10
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Updated 3/20/08 2:51 PM
they are found. In contrast, anteater morphology has evolved three times in mammals
— in the ancestors of echidnas, true anteaters, and pangolins. There are very few
species in each of these groups (four species of echidna in New Guinea and Australia,
four species of anteater in South America, eight species of pangolin in Africa and
southeast Asia). Once that combination of characteristics evolved there was little
opportunity for later diversification.
Species selection determines the relative numbers of species with different
adaptations. There are more species of bats than species of anteaters because of
species selection. But the adaptations evolved in the ancestors of each group because
of natural selection. Wings in species ancestral to bats evolved because wings were
favored by natural selection. A long nose and long tongue evolved in species
ancestral to echidnas because longer noses and longer tongues were favored by
natural selection.
 Sample questions
 Which statement best explains why there are more species of bats than there are
species of anteaters?
a. Bats evolved long before anteaters.
b. Natural selection did not favor the evolution of wings in bats.
c. Flying created many opportunities for speciation.
d. Flying prevented gene flow.
e. Anteaters evolved only in South America.
 Which statement best provides the best definition of heterochrony?
a. A change in the timing or rate of an organism’s development.
b. A change in the time of mating.
c. A change in life span.
d. A loss of radial symmetry of flowers.
e. The differentiation of repeated structures.
 The bithorax mutation in Drosophila melanogaster is an example of which one of the
following?.
a. Paedomorphosis.
b. Exaptation.
c. Species selection.
d. An advantageous mutation.
e. A mutation of a Hox gene.
 The three bones that make up the inner ear of mammals illustrate which evolutionary
principle?.
a. Reproductive isolation evolves in the absence of gene flow.
b. Evolutionary novelties can arise from existing structures
c. Species selection is responsible for many evolutionary novelties.
d. Genetic drift can cause rapid changes in allele frequency.
e. Large morphological changes can result from small changes during
development.
Correct answers: c, a, e, b